▄Anisotropic Crystals▄: It will also be found that nearly all sections (the exceptions being given later) of crystals in the remaining five systems, produce quite a different effect on transmitted light. In these crystals the velocity of transmission of light varies with the _vibration direction_ of the light rays. This property, called _double refraction_,[8] seems to result from the power of resolving a ray of ordinary light, with ether vibrations in all directions, into two rays with ether vibrations in planes at right angles to each other; the two resulting rays traversing, usually, divergent paths in passing through the section. The mineral calcite (Iceland spar) exhibits this property to a marked degree, and in certain sections will show a _double image_, Fig. 1. That the vibration directions of the two doubly refracted rays are in planes at right angles to each other, can be easily proved by using a nicol prism.[9] In most cases the separation of the two images is so slight as not to be perceived by the eye, and the practical method of testing a crystal section for double refraction will be given later, p. 27. The crystals that show double refraction are further divided into two groups, _uniaxial_ and _biaxial_: (1) ▄Uniaxial▄, or those in which the optical characters are symmetrical to _one_ direction, called an optic axis. This _optic axis_ is the crystallographic vertical axis, _ć_; and parallel to this direction there is a single value only for the light velocity and no double refraction takes place.[10] Hence any section parallel to the base (001) being at right angles to the optic axis, acts like a section of an isotropic crystal and transmits all the perpendicularly incident rays of light with no change. In any other section double refraction takes place and it can be proved by using a nicol prism that the two rays have ether vibrations, one in the plane passing through the incident ray and the _ć_ axis of the crystal, and the other in a plane at right angles thereto, hence in the basal plane. This latter ray, which has a constant velocity, is called the _ordinary ray O_; and the other ray, with velocity varying with the inclination of the section to _ć_, is called the _extraordinary ray E_.[11] The vibration directions are either parallel or symmetrical to cleavage cracks and crystal outlines. In sections parallel to the optic axis, the two doubly refracted rays have the maximum difference in velocity of transmission, and hence their vibration directions are called _principal vibration directions_[12] and the plane containing them an _optical principal section_. In these sections the refractive index of the ray vibrating parallel to _ć_ (extraordinary ray) is denoted by ε, and that of the ray vibrating parallel to the basal plane (ordinary ray) by ω.[13] To this group belong all _Tetragonal_ and _Hexagonal_ crystals.